NASA are playing with lasers again and this time, they have more on their mind than finding phytoplankton or searching for life on Mars.
Their mission, ICESat-2 is to create the most detailed map of Earth ever made in order to track the changes to the surface of the planet. In particular, the mission is interested in the changes to the polar ice caps. They hope to achieve this using ATLAS, a photon-counting laser altimeter that will be able to calculate the distance between the surface of the Earth and the satellite to within 2 inches by timing the return of the photons.
And so, they have developed a stopwatch that can measure a billionth of a second from a satellite in space.
What is ATLAS?
ATLAS, also known as the Advanced Topographic Laser Altimeter System, is made up of two lasers (one for backup) that fire at 10,000 pulses per second. Such a fast pulse rate means that ATLAS can take a measurement every 2.3 feet along the ground path of the satellite.
When the photons are released, they travel through a series of lenses before being beamed to the ground. This pathway is to start the stopwatch as well as check the wavelength of the laser (532 nm, bright green on the visible spectrum), to ensure that the laser and telescope are perfectly aligned and set the size of the ground footprint. The pathway also splits the laser into 6 beams.
Phil Luers explains more:
Measuring a Billionth of a Second
With each laser pulse, 20 trillion photons leave ATLAS but only about a dozen will return to the satellite’s telescope. Catching those photons is vital for taking the required measurements and this is what the beryllium telescope is for.
Tyler Evans explains the process in this video.
The primary dish is 80cm in diameter and is a curved reflective dish which focuses the laser beams onto the secondary mirror. This reflects the light to the centre of the back of the telescope where the detectors are. The telescope is made out of beryllium, a material with a high strength to weight ratio. This means that although it is relatively light at a mere 50lbs, it is still very strong.
Once the photons have been through the telescope they go through optical filter assemblies or Etalons, which only have a bandwidth of 532 nanometers – exactly the same as the laser. This filters out any photons coming from the sun. The photons then go to the PMT detectors, Photomultiplier Tubes, where the photons are converted to electricity and timed.
The detector then analyzes the photons that have come back from Earth compared to those sent out. This then checks the stopwatch and the elevation of the land is determined by measuring the time it has taken for the photons to return to the telescope.
The telescope is aligned using the Laser Reference System, designed and built by the ATLAS engineers. This system uses 4 lasers firing from the centre of the telescope that are bent around a periscope and into the LRS (Laser Reference System Camera) which knows from tracking the four spots how the telescope is positioned. The telescope also has a Star Tracker to see how the satellite is pointed relative to Earth based on the constellations it sees.
Using the LRS, the satellite’s position can be calculated within 5 metres – or as NASA put it with ‘ridiculous accuracy’. Knowing exactly where the telescope is located is vital for calculating where the land below it is and therefore, which area they are currently mapping and how the telescope should be positioned.
Making the Most Detailed Map of Earth
A return trip between ATLAS and Earth takes a mere 3.3 milliseconds – give or take a millisecond. Using this and the satellite’s position, the researchers can work out the distance the photon has travelled. To determine the elevation, the computer program creates a photon clouds that show thousands of data points.
Sunlight in the 532 nm range will also be detected by ATLAS’ photon detectors, but by applying another computer programme to search for the stronger signals in background clouds, scientists can “determine the elevation of ice, land, water and vegetation below ICESAT-2”. Essentially, they can remove the background noise to concentrate on the signal they are searching for.
What’s Next for ICESat-2?
The ICESat-2 is scheduled to be launched into orbit in 2018. Not only will they be able to get a more accurate picture of the changes to the polar ice caps, they will also be able to measure the amount of vegetation that makes up an area’s biomass. This will inform their calculations of the amount of carbon forests take in or release into the atmosphere.
While operational, ICESat-2 will collect a tetrabyte of data every day, creating a dense grid of measurements across the world. This will inform scientists of all sorts of changes to our planet, from measuring movements after an earthquake or volcanic eruption to the effects of avalanches. In other words it will provide a real wealth of knowledge.
We can’t wait to see what the results are.
